Silica-based optical fiber is widely used in optical telecommunications, where it constitutes the currently preferred transmission medium. Such fiber also has other uses, e.g., in sensor applications, or as an optical gain medium.
Optical fibers comprise a core and at least one cladding, with the cladding having an index of refraction lower, at least in part, than the effective index of refraction associated with the core. Both multimode fibers and single mode fibers are routinely fabricated.
Dopants used in silica-based communications fibers include germania, an index-raising dopant, which is currently the principal and most widely used dopant, as well as other dopants such as phosphorus and other index-raising dopants, and the index-lowering dopants, fluorine and boron. Boron and phosphorus are also used to improve processing characteristics, such as those associated with sintering, and B and possibly others, can be used to produce anisotropic stress. Ge is a relatively costly dopant of limited abundance in nature, and it would be desirable to find acceptable less expensive and more abundant alternative dopants, especially for multimode communications fibers.
The use of alumina as a dopant has been specifically investigated, and although the presence of higher concentrations (e.g., greater than about 5 mole %) of alumina in vitreous silica was known to generally result in devitrification, it has recently been disclosed that co-doping with phosphorus permits alumina doping in excess of 5 mole % without devitrification. See, U.S. patent application Ser. No. 527,970, (now U.S. Pat. No. 4,616,901) a continuation-in-part of U.S. patent application Ser. No. 367,091, filed Apr. 9, 1982, (now abandoned) co-assigned with this, and incorporated herein by reference.
The literature is replete with suggestions of various dopants for use in the fabrication of optical fibers. However, despite the fact that the list of disclosed possible dopants of silica is long, the number of elements that have actually been found useful for producing high-silica optical fiber by a vapor phase deposition process, especially elements that can be incorporated at relatively high levels, typically higher than about 0.2 mole %, is really quite small. These elements include, in addition to the aforementioned elements Ge, B, Al, P, and F, also Zr and, possibly, Pb and Sn.
Several general techniques for producing optical fiber are known, but the currently most frequently used general technique, and the only one of interest herein, involves formation of glass by the reaction of one (or more) gaseous glass forming precursor compounds and one (or more) oxidizers at a relatively high temperature, e.g., 1800.degree. C., and deposition of the amorphous reaction product onto a substrate. The reaction can take place in a confined space, e.g., inside a silica substrate tube, or it can take place in an unconfined manner, e.g., in or near the flame of a O.sub.2 /H.sub.2 torch. Among the former methods are the modified chemical vapor deposition (MCVD) method, and various plasma deposition methods. Among the latter methods are the vapor axial deposition (VAD) method and the outside vapor phase oxidation (OVPO) method. These techniques ae well known to those skilled in the art, and do not need detailed review here. See, for instance, J. B. MacChesney, Proceedings of the IEEE, Vol. 68(10), (1980), pp. 1181-1184; T. Izawa et al, ibid, pp. 1184-1187; P. C. Schultz, ibid, pp. 1187-1191. We will refer to these processes herein collectively as vapor phase deposition (VPD) methods.
All of the VPD methods have the potential of producing glass of very low impurity content, and it would be highly desirable if the range of dopant elements that can actually be used in such processes could be increased from the presently small number. For instance, the ability to incorporate more than minor amounts of 4f-type rare earths into silica would be of considerable interest, e.g., for sensor or laser applications, and possibly to reduce radiation sensitivity of communications fiber. However, attempts in this direction have typically not been successful. For instance, H. Namikawa et al, Japanese Journal of Applied Physics, Part 2, Vol. 21(6), pp. L360-L362 (1982), indicate that silica glass containing 0.25 mole % and more, of Nd.sub.2 O.sub.3 shows opalescence. Similarly, D. A. Thompson et al, Proceedings of the Society of Photo-Optical Instrumentation Engineers, August 1984, San Diego, Calif., pp. 170-172, report that silica glass containing 0.4 wt. % CeO.sub.2 opalesces, the opalescence being due to the presence of droplets of a second phase. It is probably obvious that glass that opalesces or is otherwise not single phase is unsuitable for many lightguide applications (e.g., as communications fiber) due to the generally unacceptably high scattering losses associated with such structure.
The rare earths are not the only group of elements that contain potentially useful dopants for optical fiber applications. For instance, the alkalis and alkaline earths also are of potential interest.
Not only can incorporation into vitreous silica of many of the above elements lead to devitrification or opalescence, it is also found that halogen compounds of these elements, the types of compounds typically used as gaseous reactants in VPD processes, frequently have a relatively low vapor pressure. This low vapor pressure typically makes impractical the use of these compounds in standard VPD glass forming processes.
A cerium compound that can be useful as a precursor material in VPD has been disclosed in the above cited paper by D. A. Thompson et al. The compound is a volatile and thermally stable beta diketonate complex of cerium that probably cannot easily be prepared in situ. However, it would clearly be desirable to have available means for easily and simply producing in situ high vapor pressure halogen compounds of the potentially useful silica dopants, since this would facilitate the production of optical fibers of novel compositions by standard VPD processes.
A method for producing multicomponent glass fiber preforms by a modified VPD process is disclosed in U.S. Pat. No. 4,336,049. The patent teaches that gaseous glassforming precursor SiCl.sub.4 (and, if desired, one or more of GeCl.sub.4, POCl.sub.3, BBr.sub.3), mixed with a carrier gas is fed into a mixing region, that an aqueous solution of one or more metal salts (alkali metals, alkaline earths, lead, or lanthanum) is nebulized and the thus formed aerosol also introduced into the mixing region, were the mixture is reacted in an oxygen/hydrogen flame. The glassy reaction product is deposited on an adjacent substrate.
The literature is replete with information on multicomponent glasses comprising silica, including glasses comprising rare earths, alkalis, alkaline earths, and other elements of interest herein. However, these prior art glasses generally contain much lower percentages of silica (and therefore have such lower softening and working temperatures) than the glasses of concern herein. However, see, for instance, F. Ya. Galakhov et al, The Soviet Journal of Glass Physics and Chemistry, Vol. 6(1), pp. 34-37 (1980), who report on phase separation in the silica-rich part of the SiO.sub.2 -Nd.sub.2 O.sub.3 -Al.sub.2 O.sub.3 system, and indicate that Al.sub.2 O.sub.3 is a homogenizing agent of liquid-liquid separated glass of the type RO(R.sub.2 O)-SiO.sub.2. Futhermore, prior art multicomponent glasses generally are not produced by a VPD process. See, for instance, K. J. Beales et al, Processing of the IEEE, op. cit., pp. 1191-1194. Thus it appears unlikely that such glasses could be produced economically and conveniently in the high purity typically required for commumications grade and other fibers.
The methods used to produce the prior art multicomponent glasses typically do not make as severe demands on the "glassiness" (see, H. Rawson, Inorganic Glass-Forming Systems, Academic Press, page 74, for a discussion of this term) of the compositions, as do at least some of the standard VPD processes. For instance, outside deposition processes, such as VAD and OVPO, are thought to demand use of glass having high "glassiness", since these processes result in formation of a porous body that has to be dehydrated and sintered to form therefrom a homogeneous low loss glass. This dehydration and sintering requires maintenance of the body at a high temperature, e.g., 700.degree.-1450.degree. C., for a substantial period of time, typically many hours. It is this heat treatment which frequently results in phase separation and/or crystallization. However, similar phenomena have also been observed in glass produced by MCVD, a process that does not require the extended heat treatment described above.
Silica-based optical fiber, produced by means of a VPD process and containing substantial amounts of alkali metal, alkaline earth or rare earth is of substantial technological and scientific interest, due, inter alia, to potentially significant cost saving over Ge-doped fiber, possible novel sensor applications, as an optical gain medium, as a radiation-hardened fiber, birefringent fiber, and because the presence of Ge in the core is correlated with the presence of loss in fibers exposed to H.sub.2. We are disclosing herein fiber compositions that can contain significant amounts of one or more of these elements. We are also disclosing a technique for producing such fiber economically and conveniently, by delivering the substituent as an easily formed high vapor pressure compound to the reaction zone.